Check vkGetInstanceProcAddr works with NULL instance

Check vkGetInstanceProcAddr works with NULL instance

This commit makes sure vkGetInstanceProcAddr can resolve itself when
used with a NULL instance, being a platform function.

In addition, vkEnumerateInstanceVersion is always checked, when present
in the corresponding API table, to work as a platform function returning
a function pointer when called with a NULL instance.

Affected tests:
dEQP-VK.api.version_check.entry_points

Components: Vulkan
VK-GL-CTS issue: 2278

Change-Id: Ia6e23b8aa2328646003ada8e4d5947656566ccaf